So-called “general-purpose plastic” such as polyethylene (hereinafter, referred to as “PE”) polypropylene (hereinafter, referred to as “PP”), polystyrene (hereinafter, referred to as “PS”), polyvinyl chloride (hereinafter referred to as “PVC) are very popular material as various daily-use products (such as bag, various wrapping, various container, sheets, etc.) industrial parts of automobiles, electronic products, daily-use products, general merchandise, and the like, not only because they are very low in price (100 yen or less per kg) but also because they can be easily molded, and are light in weight (one severalth of these of metal and ceramics).
However, the general-purpose plastics are disadvantages in terms of mechanical strength and heat tolerance. Because of this, the application of general-purpose plastics is limited today, because the general-purpose plastics cannot satisfy the property requirements for various industrial products such as mechanical products such as automobiles, electric, electronic, and information products. For example, PE has a softening temperature of generally about 90° C. PP, which is relatively heat tolerant, is softened at about 130° C. in general. Moreover, PP is insufficient in transparency compared with polycarbonate (hereinafter, referred to as “PC”), and polyethylene terephthalate (hereinafter, referred to as “PET”). The insufficient transparency prevents PP from being used as optical products, bottle, and transparent container.
On the other hand, so-called “engineering plastics” such as PET, PC, fluoroplastics (Teflon (Registered Trademark) etc.), nylon, polymethylpentene, polyoxymethylene, acrylic resin, are excellent in mechanical strength, heat tolerance, transparency, etc. The engineering plastics, which are not softened at a temperature of 150° C. in general, are used as various industrial product materials and optical materials, which require high property, for automobiles, mechanical products and electric products. On the other hand, the engineering plastics are significantly disadvantageous. For example, they are very expensive (several hundred yen/kg to several thousands yen/kg). Further, the engineering plastics are very environmental unfriendly because monomer recycle thereof is difficult or impossible.
Therefore, large improvement of the general-purpose plastics in material properties such as mechanical strength, heat tolerance, and transparency, etc. would allow the general-purpose plastics to replace the engineering plastics, and further the metal materials. This will significantly reduce the cost of various industrial products and daily-use products, which are made of polymer or metal. Further, this will give light weight to the various industrial products and daily-use products, thereby attaining great reduction in energy consumption and great improvement in handling. For example, PET is used in bottles for drinks such as soft drinks. If PET is replaced with PP, the cost of the bottle can be reduced significantly. Moreover, the monomer recycle of PET is possible but not easy. Thus, used PET bottles are cuts in pieces and reused once or twice for low-quality purposes such as cloths fibers, films, etc., and then discarded. On the other hand, PP is easy to perform monomer recycle and can be completely recycled. The use of PP can reduce consumption of fossil fuel and CO2 production.
In order to improve the general-purpose plastics in properties such as mechanical strength, heat tolerance, transparency, etc. to replace the engineering plastics and metals with the general-purpose plastics, it is necessary to improve PP and PE in crystal ratio (crystallinity) significantly, and preferably to produce “crystal” which is a pure crystal contain almost no amorphous PP or PE. Especially, PP is higher in mechanical strength and heat tolerance than PE. Thus, advantage of PP is highly expected. Annual production of PP is increased steadily by several % each year.
One method known to improve the crystallinity of polymer is to cool the polymer melt at a slow rate. This method, however, cannot attain sufficient improvement in the crystallinity at all. Further, this method leads to significant deterioration in productivity of the product and low mechanical strength due to huge crystal particle size. Another known method is to cool the melted polymer solution under high pressure in order to increase the crystallinity. This method, however, need pressuring the polymer melt at several hundreds atm or greater. Thus, this method is only experimentally possible, but not feasible in industrial production due to complicate production apparatus design and high production cost. Thus, this method is difficult to adopt practically. One another known method to improve the polymer crystallinity is to add a nucleating agent in the polymer melt. However, this method currently so disadvantageous that it cannot avoid (a) contamination with the nucleating agent as impurity (b) increase in cost due to higher cost of the nucleating agent than resin. In conclusion, there is no method to improve the crystallinity of the polymer in the general-purpose plastic and to produce the “crystal” of the polymer at this moment.
Incidentally, many study showed that a polymer melt (isotropic melt) in which molecular chains take random conformation (so called “random coil”) is crystallized with a mixture of shish crystal form and kebab crystal form when crystallized under shear flow (see Non-Patent Citation 1). The shish crystal form is fiber-like crystal of several μm in thickness and is formed along the flow. The kebab crystal form is a lamination of thin-film crystal and amorphous skewered through the shish crystal form. This form is referred to as “shish-kebab” like Japanese Yakitori's stick and meat.
In the production of shish-kebab form, only the shish form is created locally in an initial period. The shish form has Elongated Chain Crystal (ECC) structure in which straightly-elongated molecular chains are crystallized (see Non-Patent Citation 5). On the other hand, the crystal portion of the kebab form has a Folded Chain Crystal (FCC) structure in which the molecular chains are folded at a surface of the thin-film crystal. How the shish-kebab form is produced has not been explained in terms of molecular theory, because it has not been studies kinetically. The FCC is a thin-film crystal (called lamellar crystal) which is most widely seen among polymer crystals. Moreover, it is widely known that injection molding forms a “skin” (which is a thin crystalline film of several hundred μm thickness) on surface, and a “core” inside. The core is a “lamination structure (lamination lamellar structure)” in which a folded chain crystal and amorphous are laminated. (see Non-Patent Citation 6). The skin formed from shish-kebab form but the shish is formed only sparsely. Production mechanism of the skin structure has been totally unknown in the lack of kinetic study thereon.
The inventors of the present inventions are pioneers to study the production mechanism of the shish form kinetically, and found the mechanism of the local formation of the shish form in the melt: at a boundary with heterogeneity, some molecular chains in the melt attain liquid crystal orientation because the molecular chains are elongated due to “topological interaction” with the boundary, and the melt become “Oriented melt”. (See Non-Patent Citations 2 and 3 for example.) Here, the topological interaction is an effect that string-like polymer chains pull each other because they are entangled. The topological interaction is well-known as a characteristic interaction among polymers. The inventors of the present invention are first to report the topological crystallization mechanism theory of polymers, explaining how the ECC and FCC are formed. This theory is called “sliding diffusion theory” and recognized worldwide. (See Non-Patent Citation 7.)
Moreover, the inventors of the present invention reports the general theory of the mechanism how the crystallization under low shear rate elongates the molecular chains to form the oriented melt thereby to speed up nucleation and growth speed. (See Non-Patent Citation 4.)
Based on these, it is expected that the polymer crystallization will be facilitated and high crystallinity can be achieved if the whole polymer melt become oriented melt. Here, the polymer melt become oriented melt wholly is referred to as bulk oriented melt. Further it is expected that if the bulk oriented melt can be crystallized with the orientation, “crystal” in which majority (preferably 50% or more) of the polymer molecular chains are oriented can be produced. (The crystal is referred to as bulk “polymer oriented crystals”.) In this case, the nucleation is significantly facilitated and a vast number of nuclei is created between molecular chains without adding nucleating agent thereto. This eliminates the need of the addition of the impurity and allows the crystal size to be in nano meter order. It is expected that this leads to polymers with high transparency and dramatic improvement in mechanical strength and heat tolerance.
[Non-Patent Citation 1]
    A. Keller, M. J. Machin, J. Macromol. Sci., Phys., B2, 501 (1968)[Non-Patent Citation 2]    S. Yamazaki, M. Hikosaka et al, Polymer, 46, 2005, 1675-1684.[Non-Patent Citation 3]    S. Yamazaki, M. Hikosaka et al, Polymer, 46, 2005, 1685-1692.[Non-Patent Citation 4]    Watanabe, et al., 2005 Annual Meeting of the Society of Polymer Science, Polymer Preprints, Japan, 54(1), 626[Non-Patent Citation 5]    B. Wunderlich, T. Arakawa, J. Polym. Sci., 2, 3697-3706 (1964)[Non-Patent Citation 4]
Terumi FUJIYAMA, “Structure of Skin Layer of Extruded Polypropylene”, Polymer Preprints, 32(7), PP 411-417 (1975)
[Non-Patent Citation 7]
    M. Hikosaka, Polymer 1987 28 1257-1264
As described above, it is theoretically expected that bulk polymer oriented crystals can be obtained by preparing a bulk oriented melt from a polymer melt and then crystallizing the oriented melt by cooling. Non-Patent Citations 2 and 3 discloses critical strain rate (“critical shear strain rate γ*”) at which oriented melt can be produced partly under shear flow. However, bulk oriented melt cannot be produced by applying shear to the polymer melt at strain rate equal to or greater than the critical shear strain rate, which produces the oriented melt only partly in the vicinity of the boundary with the heterogeneity. (In this case, the strain rate is a critical strain rate under shear deformation, that is, under shear flow. Meanwhile, later-described ε* is a critical strain rate under elongation flow.) Here, γ*=about 0.3s−1, which is smaller by one digit than the later described ε*=about several ten s−1. Thus, the bulk polymer oriented crystals cannot be obtained even if the information based on the critical shear strain rate recited in the Non-Patent Citations is used. That is, no method has not been established, which determines the critical strain rate (critical elongation strain rate), which can straighten the polymer in the polymer melt in the bulk oriented melt. Moreover, even if the critical elongation strain rate can be determined, the critical elongation strain rate varies depending on type of the polymer, polymerization rate of the polymer, and molecular weight distribution in the polymer, and entangling density, melting temperature, and the like. Thus, the critical elongation strain rate should be determined for each polymer. Thus, at this moment, the bulk polymer oriented crystals cannot be produced by applying the technologies described above.
An object of the present invention is to establish method and means for determining the critical elongation strain rate in the polymer melt, which method and means allows preparation of the bulk oriented melt by straitening the molecular chains in the polymer melt. Further, an object of the present invention is to improve the crystallinity of polymers by this method and means. Finally, an object of the present invention is to provide a method for producing the bulk polymer oriented crystals and bulk polymer oriented crystals produced by the same method.